This invention relates to a linear drive comprising a single-acting pneumatic piston cylinder unit, comprising at least one spring which acts against the pneumatic force and loads the piston rod in the direction of one end position, and comprising one form-locking pneumatic locking mechanism respectively for each of the two end positions of the piston rod.
From German Patent Document DE-OS 36 09 765, a linear drive is known which has a double-acting pneumatic piston cylinder unit and which has, in the area of the two cylinder ends, one form-locking locking mechanism respectively for each of the two end positions of the piston rod. In both positions, the locking takes place by means of springs which prestress the locking elements in the direction of the piston rod. The unlocking takes place pneumatically by overcoming the spring force, in which case the pressure gas can flow into the piston cylinder unit only after the complete unlocking and can set the piston rod into motion. In this manner, an operation is achieved which is particularly low with respect to wear and noise. The locking element, which is illustrated on the right-hand side in FIG. 1 of German Patent Document DE-OS 36 09 765, when the piston rod is moved out (from the left to the right) will rest on its surface under spring force until, when the end position is reached, it locks into the corresponding recess. In this fashion, it exercises a pressure force and a friction force on the piston rod which is suitable for damping possibly occurring slight vibrations.
A certain disadvantage of this linear drive is caused by its construction and concerns its relative large overall length.
Based on this known solution for a double-acting pneumatic linear drive, the invention is based on the object of providing a pneumatic linear drive comprising a single-acting piston cylinder unit, comprising at least one spring acting against the pneumatic force and comprising one form-locking pneumatic locking mechanism respectively for each of the two end positions of the piston rod, which is particularly compact, light, uncomplicated and operationally reliable, which can be adapted to different operating requirements by means of low expenditures, and which can be used in a wide vibration spectrum, as it occurs particularly in the surroundings of rocket engines.
This object is achieved by providing an arrangement comprising a linear drive, particularly for cryogenic control valves in liquid fuel lines of rocket engines, comprising a single-acting pneumatic piston/cylinder unit, at least one spring which acts against the pneumatic force and loads the piston rod in the direction of one end position, and a locking device including one form-locking pneumatic locking mechanism respectively for each of the two end positions of the piston rod.
The locking mechanisms for the two end positions are combined to form a locking link which locks from diametrical sides into one recess of the piston rod respectively and which has two control pistons which are applied to it, whereby a compact arrangement of the locking mechanism is achieved. The first control piston, which is smaller with respect to the cross-section acted upon by pressure, is rigidly connected with the locking link and is responsible for the locking in the pneumatically pressure-less position, that is, in the spring-actuated end position of the piston rod. With respect to a cryogenic control valve, this would preferably be the closed position of the valve.
The second control piston, which is larger with respect to the cross-section acted upon by pressure, relative to the locking link, is translatorily movable to a limited degree on a supporting tube which is rigidly connected with the locking link, in which case one stop exists that is fixed to the supporting tube and one that is fixed to the housing, and a pressure spring is arranged between the control piston and the locking link. Thus, the transmission of force between the control piston and the locking link takes place in a springy, that is, relatively soft manner.
The second larger control piston is responsible for the locking in the end position of the piston rod which is pneumatically acted upon by pressure, that is, which is active. With respect to a cryogenic control valve, this would preferably be the open position.
However, it is also responsible for the control of the supply and removal of pressure gas to the pressure space or from the pressure space of the piston/cylinder unit and for this purpose, is coupled with an additional valve piston. By means of corresponding flow ducts in the area of the valve piston, of the second control piston, of the supporting tube and of the locking link in connection with the various stops, it is achieved that the admission of pressure for activating of the piston cylinder unit takes place only after the complete unlocking of the pressure-less end position, whereby a method of operation is obtained that saves material and is largely free of jamming.
During the transition from the active into the passive position, after the unlocking of the active end position, the valve piston exposes an additional cross-section of flow between itself and the second control piston so that the movement of the piston rod takes place relatively fast.
The arrangement according to the invention has the result that each of the two control pistons is responsible for the locking in the one end position and for the unlocking in the opposite end position. The activating of the control pistons takes place actively by the one-sided pneumatic pressure admission. During every movement of the piston between the end positions, the locking link rests on one side under pneumatic or spring force against the surface of the piston rod and dampens possibly occurring vibrations by the exercised normal and frictional force.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.